1. Field of the Invention
The present invention relates to an image forming apparatus that performs image formation by an electrophotographic method, and a control method therefor.
2. Description of the Related Art
Conventionally, in the field of an electrophotographic image forming apparatus that performs image formation by executing the steps of charging, exposing, developing, transferring, and fixing, a technique (heat fixing method) is known in which an unfixed toner image transferred onto a recording sheet is heated and fused for fixing. The heat fixing method include not only a sheet heater method and an infrared lamp method, but also a fixing roller method, which is currently most widely used, and an IH (Introduction Heating) fixing method.
In an image forming apparatus that fixes a toner image on a recording sheet by the above heat fixing method, the recording sheet shrinks after the toner image has been fixed, since moisture contained in the recording sheet is evaporated by heat during heat fixation. The recording sheet for use is generally different in characteristics and properties, depending on the use (one-side printing or both-sided printing), conditions (humidity and temperature) in which the apparatus is used, thickness, hardness, and the maker which made the sheet. The ratio of shrinkage (shrinkage ratio) is different depending on the kind and thickness of the recording sheet. It is empirically known that once the recording sheet shrinks due to heat fixation, it takes a predetermined time period (approximately 15 to 20 minutes) for the recording sheet to absorb moisture in the air to return to its original size.
In general, however, when images are formed on both sides of a recording sheet, a toner image is transferred onto one surface (front surface) of the recording sheet for heat fixation, whereafter another toner image is transferred onto the other surface (back surface) of the recording sheet, and then is heat-fixed again. Therefore, when the other toner image is transferred onto the back surface of the sheet after the toner image is transferred onto the front surface thereof, the other toner image is transferred onto the sheet in a shrunk state. This causes the problem that the images on the front surface and back surface of the recording sheet are different in size. Therefore, it is required to adjust the position of the recording sheet for image formation thereon with high accuracy according to the amount of heat-caused shrinkage. To this end, it is a general practice to change the magnification of the image according to the change in the size of the recording sheet. For example, there has been proposed an invention in which the image magnification is changed by controlling the rotational speed of the scanner motor, to thereby correct displacement of the position of the sheet for image formation.
For example, Japanese Laid-Open Patent Publication (Kokai) No. H04-288560 and Japanese Laid-Open Patent Publication (Kokai) H10-149057 proposes a technique of detecting vertical size and horizontal size of a recording sheet by an optical sensor in a conveying path, calculating the vertical and horizontal expansion/contraction ratios of the recording sheet based on results of the detection, and controlling the scanning speed of a scanning optical system based on the expansion/contraction ratios of the recording sheet to thereby change magnification of the image.
Further, Japanese Laid-Open Patent Publication (Kokai) No. 2004-351908, for example, discloses a technique of controlling the length between image data by adding an image clock for use in transferring the image data at a desired point to thereby perform the magnification correction in a main scanning direction (perpendicular to the image reading direction and is along the longitudinal direction of the photosensitive member as an image carrier), to thereby correct the image size of an image to be printed.
According to the above-described prior art, the magnification of an image to be formed is changed by switching the rotational speed of the scanner motor, and hence the rotational speed of the scanner motor is changed at intervals between adjacent pages (intervals between adjacent recording sheets) to thereby perform image formation on a page-by-page basis. In this case, during changing of the rotational speed of the scanner motor (during switching the speed), the image formation cannot be started, and hence a next image formation cannot be executed until a change in the rotational speed is completed (the speed has been switched). Although depending on the type of the scanner motor, for example, it takes about several hundred msec. to complete changing of the speed after starting the same. When the scanning speed of the scanning optical system of the image forming apparatus is switched, it takes long time before the scanning speed becomes stable after it is changed. This causes a reduced print speed to degrade productivity of the image forming apparatus, when a sheet-to-sheet interval (conveying intervals of sheets which are conveyed to the transfer position) is short. Further, it is necessary to control the sheet-to-sheet interval such that a time period taken before the scanning speed becomes stable is ensured, which makes it impossible to cope with an increase in the operating speed of the image forming apparatus.
Further, a change in the image clock for transferring the image data is can only corrects the magnification of the image data in a main scanning direction (direction orthogonal to the image reading direction). Therefore, to cope with correction of the magnification in a sub scanning direction (image reading direction), it is necessary to combine the above-described technique with the corrections of the magnification disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H04-288560, and Japanese Laid-Open Patent Publication (Kokai) No. H10-149057. In this case, the rotational speed of the scanner motor is switched, and hence, similarly to the above, there arises the problem that the productivity is lowered due to a decreased in the print speed, and it is impossible to cope with an increase in the operating speed of the image forming apparatus.